It has been suggested that the complexity and weight of electronic component designs in automotive vehicles can be improved by assembling the electronic components on substrates, e.g., on the underside of vehicle dashboards. This type of system has been termed Instrument Panel (IP) Super-Integration. In this system, it is intended that individual electronic circuit boards be first assembled on substrates, with electronic components attached thereupon. Currently used conventional printed circuit board technologies utilize either additively or subtractively defined copper, a dielectric laminate and plated through hole for interconnection. One of the drawbacks of this method is that it is expensive.
Another way is being explored by the inventors to provide three-dimensional circuits for use as part of Super-Integration Systems. It involves forming interconnected electronic substrates through the preferential chemical etching of a tri-metal layer of dissimilar metals. That is, copper patterned circuit images are carried on both sides of an aluminum foil. This copper-aluminum-copper tri-metal system could be made by cladding, lamination, or plating of copper followed by selective etch of copper to form the copper patterns on both sides of the aluminum foil. After the tri-metal is laminated on a polymeric substrate as by means of an adhesive, it needs to be selectively etched to remove the aluminum and form an electronic circuit board. Alternately, copper patterns could be selected electroplated on both sides of an aluminum foil, with or without a seed layer, and then after lamination of one of the copper patterns on a substrate, the aluminum would be selectively etched away to form an electronic circuit board. One of the advantages of this Cu--Al--Cu construction is the ability to have higher density circuit patterns by providing cross-overs of Cu-tracks. In addition, this new technique is seen capable of providing better heat dissipation through the aluminum interlayer.
In order to obtain the desired circuit pattern of such a Cu--Al--Cu construction, the aluminum needs to be selectively etched away. To provide durable circuits, it is necessary to control the etching of the aluminum carefully to have an optimum undercut. Sodium hydroxide solutions have been long used to etch aluminum. Attempts at etching the tri-metal Cu--Al--Cu described above with a conventional sodium hydroxide solution was found by the inventors, however, to result in an unacceptably large degree of undercutting of the aluminum as shown in FIG. 1. This is believed by the inventors to be based on the difference in electrochemical potential between aluminum and copper which unfavorably enhances aluminum etching rates in areas adjacent to the copper in this solution. The undercut of the aluminum was found to be so high, that the copper pads are lifted. We have now unexpectedly found a way to overcome the high undercut problems associated with such chemical etching of aluminum for tri-metal Cu--Al--Cu laminates which manufactures durable chemically milled circuits. This process involves, according to one embodiment, the selective etching of the tri-metal with a particularly defined alkaline etchant solution of sodium or potassium hydroxide which includes defined amounts of nitrates at a given temperature.
Several patents were identified after invention which disclose etchants for preparing aluminum surfaces. U.S. Pat. No. 3,957,553 discloses an alkaline etching bath for aluminum to improve its appearance and prepare it for subsequent treatment. The bath comprises an alkali metal hydroxide (15-200 g/l), a chelating agent (0.5-15 g/l) like sorbitol for suppressing alumina precipitation during etching, and an oxidizing agent (0.0001-1 g/l) such as nitrates or nitrites. U.S. Pat. No. 5,091,046 discloses a process for caustic etching of aluminum to obtain a matte finish with a solution containing free sodium hydroxide and dissolved aluminum in a 0.6-2.1 g/l ratio and an etch equalizing agent at a temperature above about 70.degree. C. The latter patent discloses total sodium hydroxide concentrations of 40-120 g/l, nitrate up to 50 g/l and nitrite up to 20 g/l. These etching solutions have the drawbacks that they would have an undesirable low etching rate with the Cu--Al--Cu tri-metals when the temperature is below 55.degree. C. and would attack plastic at temperatures above 65.degree. C. It would be desirable to have an etching solution and method for etching the aluminum from copper-aluminum laminates which would selectively etch out a portion of the aluminum while leaving sufficient aluminum to provide a structurally sound circuits, such as 3-dimentional circuits. And the etching should take place at a commercially acceptable rate. The present invention provides such an etchant and method of etching out the aluminum.